This invention relates to telecommunications networks and more particularly to measuring quality of communications, including Quality of Service provided by a packet-based telecommunications network.
Packet-based telecommunications networks are subject to a number of call path or transmission impairments. Some of these impairments differ from impairments found in circuit-based or circuit-switched networks, in that the impairments may be new to packet-based networks, or their manifestation in packet-based networks may have vastly differing characteristics. For example, in circuit networks, signal propagation delay throughout the call path is usually deterministic and remains constant once the call is set up. In packet networks, this delay is non-deterministic and may vary with congestion in or failures of various network components. In circuit networks, the loss of coded samples or other content is rare. In packet networks, packet loss may be frequent and may depend on network congestion and other factors. The quality of service perceived by the user depends in part on these and other impairments.
It is useful to measure these impairments in order to report on quality of service and to allow users, network providers, and others to determine quantitatively whether quality of service objectives are met and whether service-affecting trouble is present. Various tools have been available for measuring aspects of network performance. Packet network performance analyzers have been available, but their functions have been directed to measuring raw network performance, or network performance relating to data-carrying applications, and it is not known how to correlate such measurements to the performance or quality of the network in carrying voice or other media services.
Network performance analyzers are also available for circuit networks, and measure parameters such as loss, noise, delay, and distortion of various types. These analyzers may, for example, connect via analog or digital interfaces to individual lines, trunks, and/or carrier facilities, and may measure end-to-end or intermediate-point-to-intermediate point performance of network facilities, in either the analog or digital domains. Although these devices provide useful measures of performance of circuit networks, and although some may be adapted for use with digital networks, conventional network performance measurement systems of which the inventors are aware have not been suitable for providing useful measures of performance of digital packet networks, particularly with respect to the effects of certain impairments which may be unique to packet networks.
Further, packet networks generally require an analog-to-digital interface function, and may also require compression, encoding, and or encryption functions, to be performed in the subscriber terminal. However, this requirement is not unique to packet networks. Circuit network performance measurement tools of which the inventors are aware are adapted for connection to a line, a trunk, or a carrier facility, and therefore do not include in their measurements the performance of the subscriber terminal.
Thus, although some tools have been available for measuring aspects of network performance, it is believed that no tools have been available which are directed to measuring dimensions of performance directly relevant to the quality, of voice and, other telecommunications services provided over a packet network.
It is therefore an object of the present invention to provide a system and methods for measuring Quality of Service (QoS) or other performance of communications services provided in packet-based networks that minimize the disadvantages of the prior art.
A preferred embodiment of a communications quality measurement system constructed according to the present invention comprises a test signal transmitter, a test signal receiver, and a test signal analyzer coupled to the test signal transmitter and the test signal receiver. Preferably, the communications quality measurement system further comprises a transmitting tap or interface connected to the test signal generator for coupling the test signal generator to a network being tested, and a receiving tap or interface connected to the test signal receiver for coupling the test signal receiver to the network.
The test signal generator preferably has facilities for controllably generating arbitrary signals on at least two xe2x80x9canalogxe2x80x9d signal channels in the frequency range to be tested. For networks carrying telephone-quality voice signals, this range is typically 300-3400 Hz. The test signal receiver preferably has facilities for receiving at least two xe2x80x9canalogxe2x80x9d signal channels in the frequency range to be tested and for converting the signals into a form useable by the test signal analyzer.
In a preferred embodiment, the test signal generator, the test signal receiver, and the test signal analyzer may be implemented using a general purpose programmable computer, such as an industry-standard personal computer, including appropriate audio-frequency (AF) analog-to-digital (A-D) and digital to analog (D-A) signal conversion equipment. As of the time of filing of this application, suitable signal conversion equipment, sometimes referred to as a xe2x80x9csound cardxe2x80x9d, is readily available for personal computers as an add-on card or as an integrated portion of the computer""s main circuit board.
The transmitting and receiving taps or interfaces couple the test signal between the communications quality measurement system and the network. The communications quality measurement system may be coupled, for example, to conventional analog lines or trunks using known interface circuits. In applications where the communications quality measurement system is to be used with a digital packet network, it may be advantageous to include in quality measurements the performance of the user terminals, including any elements thereof responsible for converting content carried via the digital packet media to human-useable form. In that case, the transmitting and receiving taps are preferably suitably connected to signal leads of the user terminal carrying the content in analog form, and may be optimally connected in leads carrying the signals from the terminal microphone and to the terminal speaker. For example, if the user terminal has a conventional telephone receiver, the transmitting and receiving taps may simply be resistive voltage dividers connected to the microphone and speaker leads of the receiver.
In operation, a test signal connection is made through the network path under test between a first channel of the test signal generator and a corresponding first channel of the test signal receiver. A reference signal connection is made through a known-good or reference path between a second channel of the test signal generator and a corresponding second channel of the test signal receiver. The reference signal connection may be as simple as a jumper cable between the transmitter reference channel port and the receiver reference channel port.
The test signal generator produces an appropriate test signal selected to measure aspects of the network path under test and transmits that signal on both the test channel and the reference channel. By way of example, appropriate test signals may include, in various combinations and sequences, periods of silence, band-limited white noise, impulse or step-function signals, and continuous, interrupted, or pulsed tones at various frequencies. The test signal receiver receives the test signal and the reference signal and passes a converted version of the signals to the test signal analyzer. The test signal analyzer compares the received test add reference signals to measure and report Quality of Service or other performance aspects of the network under test.